Studies in defined populations in Western Australia have observed an excess of neurologic morbidity in children born of multiple gestations. Since multiple gestations are increasing in developed countries due to assistive reproductive interventions, and cerebral palsy in twins and triplets now contributes a larger proportion of total cerebral palsy than in the past, a large international study of this problem has been undertaken by the Neuroepidemiology Branch (NEB). Eleven datasets, from Sweden, Scotland, Oxford, Bristol, London, the Mersey region of Britain, and Victoria, Western and South Australia, Atlanta and California has been assembled by Dr. Petterson of the University of Western Australia and Ann Scher, NEB. Work has established that indeed multiple births have increased during the decade of the 1980s and 1990s, and that unlike-sex pairs have increased among total twins, consistent with the view that pharmacological and other medical interventions for conception (which techniques produce chiefly dizygotic twins) that have been associated with the increase in twinning and in higher-order multiple births. This study confirms that at given birthweights, the risk of death or CP in twins may be lower than in singletons. The two major risk factors for CP in twins are prematurity and fate of co-twin [death, CP, or intact survival].